Phosphatases will be isolated and characterized from the cytosol fractions of the guinea pig heart, using (NH4)2SO4 fractionation followed by DEAE-cellulose column chromatography. The enzyme(s) will be eluted with a linear gradient of KC1 or NaC1 (0-500 mM). A crude preparation of phosphatase will be purified by affinity-chromatography on sepharose-bound derivatives of arsanilic acid which will be a competitive inhibitor of 4-(P-amino-phenylazo) phenyl-arsonic acid. Phosphatase will be the only enzyme retained by the affinity column in the presence of Pi. The enzyme will be eluted with the phosphate buffer. The enzyme will be characterized, and its properties will be compared with those of the known enzymes (phosphatases) reported in the literature. Dephosphorylation of the contractile proteins will be carried out using the phosphatase.